Thermosetting coatings prepared from phenol-functional (PF) polyester resins and melamine-type crosslinkers have been shown to exhibit superior acid-etch resistance and a unique combination of hardness and flexibility. (See, for example, U.S. Pat. Nos. 5,245,002; 5,256,756; 5,256,757; 5,256,758; and 5,256,759). Accordingly, these coatings are especially useful as automotive coatings as well as coil coatings. Phenol-functional polyesters can also be used to react with isocyanates to form thermally-reversible polymers for hot-melt adhesive applications (see, for example, U.S. Pat. No. 5,097,010). Unlike conventional polyester resins, the PF resins are more difficult to prepare due to the presence of p-hydroxybenzoic acid (PHBA) as one of the critical components. Under ordinary reaction conditions, PHBA behaves as a mono-functional reactant since its phenol group does not react with the carboxyl groups. The resins thus require proper formulation to yield desired molecular weights. Moreover, PHBA is prone to decomposition and sublimation during the reaction, so it is especially challenging to produce clear, less colored resins. In this regard, U.S. Pat. No. 5,245,002 describes a staging technique wherein certain components are combined in an initial stage, followed by polycondensation and later addition of other components followed by completion of the polycondensation. Further, U.S. Pat. No. 5,239,018 notes that it is important to utilize p-hydroxybenzoic acid which is relatively free from basic contaminants and to control the reaction temperature so as to minimize decarboxylation and hence phenol formation.
By utilizing this prior methodology, satisfactory resins can be prepared on laboratory scales--i.e., up to about 2 kg. During scale-up of these processes however, we discovered that these processes frequently led to resins which exhibited an undesireable haziness (cloudiness). Accordingly, a ten-gallon reactor (Brighton/Trinity) was utilized for resin preparation, and the same processes as disclosed in the above references were followed. As will be shown below, the present invention provides a solution to this problem and thus provides a facile process amenable to large scale manufacture of phenol-functional polyester resins of high aesthetic quality.